The subject matter disclosed herein relates to a rotor apparatus and, more particularly, to a rotor apparatus in which bearing elements pre-load at least one of first and second rotor shafts against the other.
Coaxial helicopters are helicopters that include coaxial main rotors (or co-rotors). Coaxial main rotors are generally provided as a pair of helicopter rotors mounted one above the other on concentric rotor shafts, with the same axis of rotation, but that turn in opposite directions (contra-rotation).
In order to reach high forward flight speed, coaxial helicopters utilize coaxial main rotors that are relatively rigid. This rigidity tends to induce excessive flight loads, such as especially hub moments, to the helicopter transmissions. These loads are normally transferred by the rotor shafts through their respective bearings to the gearbox housings and the airframe. The rotor shafts are typically coupled with the gear train inside of the gearbox and any deflection of the rotor shafts may degrade gear performance. Therefore, reduction in rotor shaft deflection is desired but often limited due to the magnitude of the flight loads.
Moreover, since each rotor shaft may be supported independently by a set of bearings, each rotor shaft reacts to the flight loads as a separate system and has to be sized accordingly to do so. In addition, since flight loads vary significantly based on flight conditions, the various components associated with the rotor shaft are sized for the worst expected loads. The rotor shafts may therefore be overdesigned and thus significant contributors to the weight of the dynamic systems. Indeed, in some cases, the rotor shaft weights may reach more than 50% of the entire gearbox and in coaxial transmission configurations, these issues are magnified by the presence of the two main rotor shafts.